1. Field of the Invention
The present invention relates to a process for producing a multilayer conductor structure and more particularly, to a process for producing a multilayer conductor structure which comprises multilayer conductor patterns and has a smooth surface.
2. Description of Prior Arts
In recent years, various elements have been fabricated utilizing the micropatterns of thin films and used in the fields of electronics and optics. This is based on the recent development of thin film-forming technique, photo-lithography and etching technique. Examples of such elements include semiconductor elements such as IC and LSI, liquid crystal display elements, thin-film magnetic heads and thermal heads.
It is desired that thin-film magnetic recording heads used in magnetic recording devices such as VTR, digital-recording audiotape device and magnetic disk device be more smaller and have higher performance to compensate for the miniaturization of the devices. Particularly, the development of a magnetic head having a shorter gap length and a narrower track width is desired to enable higher density-recording. Further, it is required that the magnetic head has a higher magnetomotive force. In order to meet such requirements, the magnetic head should be of a multi-turn type and the efficiency of its magnetic circuit should be higher.
The thin-film magnetic head for magnetic recording has a basic structure which comprises a substrate such as a glass substrate or a sapphire substrate, an upper magnetic pole and a lower magnetic pole provided thereon, both being composed of a film of magnetic substance such as permalloy or sendust, and a coil composed of conductor such as Al, Cu or Au provided between the poles with insulation layers therethrough. The magnetic head is fabricated, for example, by repeating the following steps in turn: steps of forming a film of the magnetic substance and forming an insulation layer or a conductor film on the substrate by a known film-forming processes such as sputtering, vapor deposition and plating, a step of patterning the upper layer by means of resist, and a step of producing a thin film pattern by chemical ethching or dry etching, thereby to provide a lower magnetic pole, an upper magnetic pole, an electrical insulation layer, a coil, a front gap, etc., superposed in this order.
In the fabrication of an element comprising such thin film patterns in multilayer structure, a profile of recess and protrusion (steps) corresponding to the thin film pattern is liable to appear on the upper thin film even where an intermediate insulation layer is provided therebetween. This causes lowering in the precision of the lithography of thin film pattern provided on an upper layer and results in decreasing the characteristics of the resulting element.
Particularly in the fabrication of a multi-turn type thin-film magnetic head having a multilayer coil, conductor patterns serving as the coils are so laminated in the form of a multilayer that there is a difficulty in forming conductor patterns in the upper part with high accuracy, and head efficiency (recording and reproducing efficiency) is liable to lower. Further, the appearance of a profile of recess and protrusion corresponding to these coil patterns on the upper magnetic pole causes increase of magnetic resistance of the resulting magnetic head and partial shortening of a distance between the upper and the lower magnetic poles in the recessed area. This causes leakage of magnetic flux, and also results in lowering of head efficiency.
Accordingly, the formation of recess and protrusion on the surface of the upper thin film corresponding to the lower patterns in multilayer structure should be prevented in fabricating the element such as the thin-film magnetic head.
Further, the elements including the above-described magnetic head should be as small in size and thickness as possible. The multilayer coil is desired to have a higher density so as to show high magnetomotive force.
There have been proposed a variety of methods of forming a multilayer structural thin film pattern having a smooth surface thereon in the production of the multilayer coil in order to solve the aforementioned problems. For example, there is known a method in which an insulation layer is provided on the first conductor pattern and then the second pattern is formed on the insulation layer by a lift-off process or a bevel etching (chemical etching) process.
The lift-off process comprises steps of providing a layer of a lift material on the insulation layer; etching the lift material layer to form the same micropattern as that of the first conductor pattern; forming a conductor film having the same thickness as that of the first pattern on the lift material layer and the insulation layer; and removing (lifting off) the lift material layer together with the conductor thereon by a stripping agent to form the second pattern in the recessed area of the first pattern. In this process, V-shaped ditches are produced between the insulation layer and the second pattern in this process so that the surface can be hardly smoothed. Further, the process has disadvantages in that strict alignment is required in the etching step and further a treating stage for removing the lift material is complicatd.
The bevel etching process comprises providing a conductor film having the same thickness as that of the first conductor pattern on the insulation layer; further providing a photoresist layer thereon; etching the area corresponding to the central part of a protrusion of the first pattern to form a resist pattern; subjecting the protrusion of the conductor film to a bevel etching treatment under prescribed etching conditions by utilizing the resist pattern; and removing the resist to form the second pattern in the recessed area of the first pattern. This method has disadvantages in that alignment for forming the resist pattern is not easy, the suitable etching conditions can be hardly chosen and as a result, inverse V-shaped protrusions are left in the second pattern so that it is difficult to form a perfectly smooth (flat) surface.